Electrical heater de-icer

ABSTRACT

A de-icer for a structural member of an aircraft is provided. The de-icer includes first and second heaters having first and second electrically conductive strips, respectively. The first and second conductive strips are configured to form first and second marginal portions, respectively. The first and second heaters are positioned relative to each other such that the marginal portions are overlapped.

This application is a division of application Ser. No. 07/531,588, filedJun. 1, 1990, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an improved electrical de-icer forheating an ice accumulation surface of an aircraft to control iceaccumulation on a structural member, and more particularly to a de-icerfacilitating heat flow between adjoining elements.

2. Description of the Prior Art

The accumulation of ice on aircraft wings and other structural membersin flight is a danger that is well known. As used herein, the term"structural members" is intended to refer to any aircraft surfacesusceptible to icing during flight, including wings, stabilizers, engineinlets, rotors, and so forth. Attempts have been made since the earliestdays of flight to overcome the problem of ice accumulation. While avariety of techniques have been proposed for removing ice from aircraftduring flight, these techniques have had various drawbacks that havestimulated continued research activities.

One approach that has been used is so-called thermal deicing. In thermalde-icing, the leading edges, that is, the portions of the aircraft thatmeet and break the airstream impinging on the aircraft, are heated toprevent the formation of ice or to loosen accumulated ice. The loosenedice is blown from the structural members by the airstream passing overthe aircraft.

In one form of thermal de-icing, heating is accomplished by placing anelectrothermal pad(e), including heating elements, over the leadingedges of the aircraft, or by incorporating the heating elements into thestructural members of the aircraft. Electrical energy for each heatingelement is derived from a generating source driven by one or more of theaircraft engines. The electrical energy is intermittently orcontinuously supplied to provide heat sufficient to prevent theformation of ice or to loosen accumulating ice.

With some commonly employed thermal de-icers, the heating elements areconfigured as ribbons, i.e. interconnected conductive segments, that aremounted on a flexible backing. The conductive segments are separatedfrom each other by gaps, i.e. intersegmental gaps, and each ribbon iselectrically energized by a pair of contact strips. When applied to awing or other airfoil surface, the segments are arranged in strips orzones extending spanwise or chordwise of the aircraft wing or airfoil.One of these strips, known as a spanwise parting strip, is disposedalong a spanwise axis which commonly coincides with a stagnation linethat develops during flight in which icing is encountered. Other strips,known as chordwise parting strips, are disposed at the ends of thespanwise parting strip and are aligned along chordwise axes. Otherzones, known as spanwise shedding zones, typically are positioned aboveand below the spanwise parting strip at a location intermediate thechordwise parting strips. Between adjacent zones, a gap, known as aninterheater gap, exists.

In one preferred form of de-icing, an electrical current is transmittedcontinuously through the parting strips so that the parting strips areheated continuously to a temperature above 32° F. In the spanwiseshedding zones, on the other hand, current is transmitted intermittentlyso that the spanwise shedding zones are heated intermittently to atemperature above about 32° F. While this technique of heating thevarious zones generally is effective to melt ice (or prevent itsformation) without the consumption of excessive current, a problemexists in that melting of ice in the inter-segmental and interheatergaps can be difficult or impossible. Moreover melting of ice on oraround the contact strips can also be difficult or impossible.Accumulation of ice in the gaps and on the contact stripe isparticularly undesirable since the unmelted ice serves as "anchors" forice that would be melted but for the ice accumulated in the gaps or onthe contact strips.

Desirably, a thermal de-icer would be available that would provideeffective de-icing action while employing an efficient design thatminimizes the formation of cold spots.

SUMMARY OF THE INVENTION

In response to the foregoing concerns, the present invention provides anew and improved thermal de-icer for aircraft structural members. Thede-icer according to the invention includes a first heater and a secondheater, which first and second heaters respectively include a firstelectrically conductive strip and a second electrically conductivestrip. The first and second conductive strips are configured to form afirst marginal portion and second marginal portion, respectively. Thefirst and second heaters are positioned relative to each other such thatthe marginal portions are overlapped.

In one preferred embodiment of the invention, the de-icer is attached toan outer surface of the structural member. Each of the first and secondstrips communicate with a current source by way of electrical contacts.The contacts have portions that are remotely disposed of the first andsecond strips, and the contacts are heated by a portion of either thefirst heater or the second heater. Additionally, an insulating layer isused In conjunction with the heaters to facilitate uniform distributionof heat throughout the de-icer. Finally, the strips are configured tomaximize the efficiency of current flow therein, and hence improve theirability to generate heat. In the preferred form of operation, current istransmitted through the first strip and the second strip by way of thecontacts so that enough heat is generated in the marginal portions toeliminate cold spots that typically form in the marginal portions.

In the preferred embodiment, a spanwise parting strip is disposed alongan axis which typically defines the stagnation line developed undericing conditions- Chordwise parting strips are disposed at the ends ofthe spanwise parting strip and are aligned along chordwise axes. Thespanwise parting strip and a plurality of spanwise shedding zones arearranged in a spanwise shedding area.

In one preferred embodiment, the spanwise parting strip and each of thespanwise shedding zones can be individually controlled, so that thelocation of the spanwise parting strip can be shifted in accordance withflight conditions and, more specifically, with any shift in thestagnation line. While the spanwise and chordwise parting stripstypically are heated continuously to maintain an ice free condition,groups of spanwise shedding zones in the shedding area(s) are heatedsequentially to melt or loosen accumulated ice. Even though chordwiseparting strips typically are used to minimize ice bridging between theshedding areas, bridging can be alleviated or possibly eliminated byoverlapping the time interval of heating each zone with that of the nextadjacent zone.

Numerous advantages of the present invention will be appreciated bythose skilled in the art.

One advantage of the present invention is that it minimizes colds spotsdeveloped during de-icing as a result of the existence of interheatergaps. By overlapping elements, heat is effectively and efficientlygenerated in the interheater gaps. This is in contrast to the typicalthermal de-icer which is incapable of effectively generating heat in theinterheater gaps.

Another advantage of the present invention is that it optimizes elementdimensions, such as conductive segment length, conductive segment widthand inter-segmental gap width. Optimization of element dimensionspromotes efficient heating and minimizes the amount of overlapping thatis required to gain desired heat distribution in the de-icer.

Yet another advantage of the present invention is that it eliminatescold spots which can arise on and around contacts. That is, in thepresent invention, each contact is overlapped preferably with a heaterto prevent icing of the contact means. Alternatively, the contacts aresegregated from ice accumulating surfaces by, for example, folding thecontacts under the heaters. Hence the heater design serves to eliminatesubstantially all of the ice accumulation which typically occurs on andaround the contacts.

Yet another advantage of the present invention is that it improves heattransfer capability of the thermal de-icer. More specifically, throughuse of an insulating layer heat is more evenly distributed throughoutthe de-icer. At the same time, use of insulating layer enhances thede-icer's ability to transfer heat to the interheater gaps.

Another advantage of the present invention is that it affords highlydesirable levels of heating while using a minimum amount of power. Morespecifically, by sequentially heating spanwise shedding areas, powerconsumption is minimized without sacrificing de-icing capabilities.Additionally, flexible control of the elements maximizes de-icingcapability. In particular, as flight conditions change, the intervalduring which each element is heated can be varied.

The foregoing and other features and advantages of the present inventionwill become more apparent when viewed in light of the description of thebest embodiment of the invention and the drawings that follow, whichtogether form a part of the specification.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial perspective view of an airfoil having a thermalde-icer mounted along the airfoil's leading edge;

FIG. 2 is a top plan view of a prior art thermal de-icer;

FIG. 3 is a partial, broken-away top plan view of a thermal de-iceraccording to the invention mounted on a structural member;

FIG. 4 is a vertical cross-sectional view of the thermal de-icer takenalong plane 4--4 of FIG. 3;

FIG. 5 is a partial, top plan view of a prior art deicer heating elementwith sharp inside corners;

FIG. 6 is a partial, top plan view of a de-icer heating element of thepresent invention with rounded inside corners;

FIG. 7 is a partial, top plan view of a prior art angled de-icingheating element;

FIG. 8 is a computer generated representation of current flux patternsin a ribbon segment of a thermal de-icer;

FIG. 9 is a partial, perspective view of the thermal de-icer of FIG. 3mounted on a surface of an unfolded airfoil;

FIG. 10 is a timing diagram depicting a heating sequence used with thede-icer of FIG. 9; and

FIG. 11 is a partial, top plan view of one de-icer heating elementsuperimposed with two other de-icer heating elements.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention is directed toward thermal de-icers used onstructural members of an aircraft. Referring to FIG. 1, a thermalde-icer 10 according to the prior art is shown mounted on a structuralmember 11 in the form of a wing. As is known, the structural member 11includes a chordwise axis and a spanwise axis. During flight, airflowimpinges a leading edge 13 of the structural member 11, and a number ofstagnation points develop, forming a stagnation line or axis, whichstagnation line varies during flight conditions.

The de-icer 10 is mounted symmetrically about the stagnation line whichwould be most commonly encountered during icing conditions. Due to thesweep of the structural member 11 upon which the de-icer 10 is employed,a pair of chordwise disposed or side edges of the de-icer 10 have achevron shape when the de-icer 10 is flat. As will be appreciated bythose skilled in the art, configuring the side edges in this mannerallows for two of de-icers 10 to be placed side-by-side, along theleading edge 13, without forming a gap between the two de-icers 10. Fora structural member 11 with no sweep, the side edges would beperpendicular with the stagnation line when the de-icer 10 is flat. Inthe following discussion, the operation of a single de-icer 10 will bediscussed. It should be recognized, nonetheless, that commonly a numberof de-icers 10 would be mounted adjacent to one another along theleading edge 13 of the structural member 11.

FIG. 2 illustrates in further detail the prior art thermal de-icer 10which includes a plurality of elements or ribbons 12. The elements 12typically are mounted on a flexible backing 15 and are arranged toprovide a spanwise parting strip 14, chordwise parting strips 16, andspanwise shedding zones 18. Current is transmitted to the elements 12 byway of contacts 20-23. As illustrated in FIG. 2, contacts 20-23 includefour pairs of contact pads, four of which pads are disposed on one endof the de-icer 16 and the other four of which are disposed on anopposing end of the de-icer 10. In operation, voltage differences areestablished between the pad pairs so that current flows through each ofthe elements 12.

Interheater gaps 24 are disposed between the various zones 14, 16 and18. The elements 12 are defined by interconnected conductive segments26, which conductive segments 26 are aligned along axes that areparallel with either the stagnation line or chordwise axes of thestructural member 11. Each pair of conductive segments 26 isinterconnected by a turn 28 and defines an inter-segmental gap 30.

In operation, current is transmitted continuously to the spanwise andchordwise parting strips 14, 16 so that heat is generated continuouslytherein. Heat is generated continuously in the spanwise parting strip 14since ice that accumulates adjacent to the stagnation line, such as rimeice, tends to be most difficult to melt. Current is transmittedintermittently to the spanwise shedding zones 18 so that heat isgenerated intermittently therein. One object of the de-icer 10 is tomelt all of the ice that accumulates adjacent to the elements 12, but inpractice certain problems arise.

First, with heating or de-icing systems such as de-icer 10, ice canaccumulate in the interheater gaps 24 as well as in the inter-segmentalgaps 30. More specifically, during operation, very little current flowsin the outer portions or corners of the turns 28 so that even when, forexample, the turns 28 of one of the elements 12 are positioned close tothe turns 28 of another of the elements 12, there still is no practicalway to transfer heat from the one set of turns 28 to the other set ofturns

Second, in common prior art arrangements of de-icer 10, no heat issupplied to contacts 20-23. In particular, the contact pads of contacts20-23 are much wider than typical conductive segments 26 and areattached to a heavy leadwire having a relatively large cross-sectionalarea. Thus, the contact pads dissipate relatively little energy and arecold spots, upon which ice accumulates. Moreover, the contact pads serveas "anchors" for ice which would have melted but for the cold spotsgenerated by the contacts 20-23.

Third, the interheater gaps 24 between the chordwise parting strips 16and the spanwise shedding zones 18 are particularly difficult to heat.More specifically, referring to FIG. 2, the outside corners of the turns28 disposed near the chordwise parting strip 16 are angled toaccommodate for the chevron-shaped edges of the de-icer 10. Inoperation, current does not flow efficiently in these angled corners andthe resulting cold spot(s) can make the task of sufficiently heating theinterheater gaps 24 even more difficult.

Finally, some of the conductive segments 26 are too short in length toprovide adequate heating. It has been found that when the conductivesegments 26 are too short, current flux density is such that anundesirable heating pattern is achieved in the element 12.

It is believed that the de-icer 10, while certainly more efficient thanmany known thermal de-icers, is incapable of minimizing cold spots. Thatis, even if cold spots could be eliminated in the interheater gaps 24 bygenerating more heat in the elements 12, the de-icer 10 still wouldconsume undesirably high levels of power. Moreover, generation of moreheat would not necessarily allow for melting in the region of thecontacts 20-23 or in certain of the turns 28 formed near the chordwiseparting strips 16. Therefore, there is a need for a heater thateconomically and efficiently provides heat to the interheater gaps 24and the inter-segmental gaps 30 as well as to the contacts 20-23.

Referring to FIG. 3, a partial plan view of a thermal de-icer accordingto the invention is indicated by the numeral 40. It will be recognizedthat the de-icers 10 and 40 have some similar features. Indeed, both ofthe de-icers 10 and 40 are shaped similarly and are mounted on thestructural member 11 in the same manner. The following discussion shouldmake it clear, however, that the present invention includes manyfeatures that are not contemplated by the prior art.

The de-icer 40 is mounted along the leading edge 13 of the structuralmember 11. The structural member 11, in the present example, is acomposite material, but, in other examples, could be a metal, such asaluminum. Referring to FIGS. 3, 4 and 9, the de-icer 40 includes aspanwise parting strip 44, a chordwise parting strip 45 and spanwiseshedding zones 46, each of which is mounted on either a flexible backing47 or a flexible backing 48. The spanwise parting strip 44 preferably ismounted along an axis which is coincidental with a stagnation line mostcommonly encountered during icing conditions. The strips 44, 45 and thezones 46 include conductive elements or ribbons 50 which are positionedalong either a spanwise or a chordwise axis. The elements 50 preferablyare configured in serpentine patterns.

Current is transmitted to the elements 50 by way of contacts 51, whichcontacts 51 are connected to the elements 50. In the preferredembodiment, contacts 51 include pairs of contact strips or pads, each ofwhich strip is connected to an end of element 50 and includes asubstantial portion disposed remotely of strips 44, 45 and zones 46.Only one contract strip is shown for each of the elements 50 in FIG. 3,but it can be appreciated that each element 50 has two ends and each endhas a contact strip attached thereto. While operation of the de-icer 40will be discussed in further detail below, it can be recognized thatheat is generated by each element 50 when current is transmittedtherethrough by establishing a voltage differential between itscorresponding pair of contact stripe.

Each of the elements 50 (FIGS. 3 and 6) includes a plurality ofstraight-sided conductive segments 52 having a uniform width. The sidesof most of the conductive segments 52 are perpendicular to either aspanwise axis or a chordwise axis. Generally, each conductive segment 52is flanked by parallel conductive segments 52, but some of theconductive segments 52 are perpendicular to adjacent conductive segments52. Perpendicular positioning of a conductive segment 52 relative to anadjacent conductive segment 52 permits the conductive segments 52 to beshifted from a vertical orientation to a horizontal orientation, or viceversa. Each pair of parallel conductive segments 52 is joined by a turn54 and includes an inter-segmental gap 56.

The elements 50 are separated from one another by interheater gaps 58.In the preferred arrangement of FIG. 3, a number of interheater gaps 58have been eliminated by superimposing an edge or marginal portion ofchordwise parting strip 45 with edge or marginal portions of thespanwise parting strip 44 and the spanwise shedding zones 46. Moreover,referring to a lower section of the de-leer 10 in FIG. 3, one of theinterheater gaps 58 between spanwise shedding zones 46 has beeneliminated by overlapping turns 54 of one of elements 50 with the turns54 of another of elements 50. The significance of overlapping theelements 50 will be discussed in further detail below. It can beappreciated at this moment, however, that such overlap eliminates coldspots which can exist in interheater gaps 50 during the heating ofelements 50, and facilitates more desirable heat distribution betweenelements 50.

As mentioned above, cold spots, which can function as ice anchors,commonly form in the area covered by the contacts 51. In the preferredembodiment of FIG. 3, the local cold spots attributable to the contacts51 are eliminated by overlapping the contacts 51 with the chordwiseparting strip 45. Under one alternative technique for eliminating coldspots attributable to the contacts 51, the contacts 51 are folded underthe elements 50 subsequent to mounting and etching of the elements 50and contacts 51 on either of backings 47, 48. Under another alternativefor eliminating cold spots, the contacts 51 are overlapped by a spanwiseparting strip 44 or a spanwise shedding zone 46.

When the de-icer 40 is attached to an upper surface of structural member11, leadwires are coupled to contacts 51 from the underside of de-leer40. During installation leadwires are extended from the electricalsystem of the aircraft and through the leading edge 13 to the contactmeans 51. It also can be appreciated that chordwise parting strips 45have contacts (not shown) which in one embodiment can be disposed underportions of the one or more spanwise parting strips 44.

In FIG. 4, the de-icer 40 is attached to an outer surface of thestructural member 11. In other embodiments, however, the de-icer 40could be attached to an inner surface of the structural member 11. Thede-icer 40 of FIG. 4 has an erosion-resistant layer 64 which serves asthe surface upon which ice accumulates. While in the preferredembodiment erosion-resistant layer 64 is a stainless steel erosionshield having a thickness of about 0.005 inch, in other embodiments theerosion-resistant layer 64 could be constructed of a thin layer ofthermoplastic film, polyurethane paint, elastomeric material, titanium,aluminum, nickel or other durable materials.

Referring still to FIG. 4, the element 50 of the chordwise parting strip45 is spaced from the erosion resistant layer 64 by layers 66 and 68 ofepoxy film adhesive. The element 50 of the chordwise parting strip 45 isseparated from the element 50 of the spanwise shedding zone 44 by alayer 70 of cured epoxy/fiberglass which is sandwiched between layers 72and 74 of epoxy film adhesive. In one embodiment, the epoxy filmadhesive layers 66, 68, 72 and 74 are rated at 0.03 pounds per squarefoot. The element 50 of spanwise shedding zone 46 and the adjacentcontacts 51 are supported by epoxy layer 76. The layer 76 isprepregnated with fiberglass. In the preferred embodiment, the layers 70and 76 serve to provide backing and support for elements 50 and contacts51. In one example, elements 50 and contacts 51 are made ofcopper-aluminum alloy as well as mounted and etched on either backing 47or 48. The etched copper-aluminum strips may range in thickness from0.001 inch to 0.010 inch. Additionally the backing 47, formed of layers70, 72 and 74, has a thickness ranging from about 0.002 inch to 0.010inch.

When the de-icer 10 is mounted on the outer surface of the structuralmember 11, as in FIG. 4, both the chordwise parting strip 45 and thespanwise shedding zone 46 preferably are underlain by insulating layer78 to facilitate heat distribution in two dimensions. The insulatinglayer 78 is preferably spaced from structural member 11 by a backing 79of fiberglass prepregnated with epoxy. In the preferred embodiment, theinsulating layer 78 is a resinous matrix which is impregnated withhollow glass spheres. A product which serves as insulating layer 78 inthe preferred embodiment is manufactured by Hysol, and sold under thetrademark SYNCORE. In the present example, the insulating layer 78 isabout 0.04 inch thick.

It has been found that use of the insulating layer 78 serves to enhancethe heat transfer characteristics between the stripe 44-45 and the zones46, and, more particularly, promotes distribution of heat throughout thede-icer 40. Referring again to FIG. 4, it will be recognized by thoseskilled in the art that the elements are capable of directing heattoward the structural member 11. If insulating means 78 were not used,an undesirable amount of generated heat could be transferred to thestructural member 11. When insulating layer 78 is employed, heatdirected toward the insulating layer 78 is distributed throughout thede-icer 40 rather than being undesirably transferred to the structuralmember 11.

Referring now to FIGS. 4-8, the configuration of each element 50 isexplained in further detail. In the preferred embodiment, each turn 54(FIG. 6) includes a pair of inside corners 82 and a pair of outsidecorners 84. To determine the heat distribution generated in conductivesegments 52 and turns 54 as a result of current flux pattern in theelement 50, computer-generated analyses were performed. It will beappreciated by those skilled in the art that current flux lines, whichare indicated in FIGS. 5-7 by the numeral 83, serve to depict thedistribution of current flow across a cross-sectional area that istransverse to either a conductive segment 52 or a turn 54. Heatgeneration varies directly as a function of the spacing of the fluxlines 83 to one another. That is, as the spacing between adjacentcurrent flux lines 83 is reduced heat generation increases, and viceversa.

In one computer-generated analysis, it was found that when insidecorners 82 are relatively sharp (FIG. 5), the current flux lines 83around inside corners 82 are squeezed so that hot spots and cold spotsare developed along the inside portion of turn 54. In comparison, whenthe inside corners 82 are rounded or curved as in FIG. 6, the flux linesare more evenly spaced so that heat is more uniformly distributed aboutthe inside portion of the turn 54. Many prior art embodiments use sharpinside corners within element turns, as in FIG. 5, apparently failing torecognize the advantageous current flow that can be achieved by usingrounded inside corners.

Referring to FIG. 7, an angled element 85 used in prior art arrangementsis shown. In swept airfoils, elements, such as element 85, have beenconstructed to follow airflow streamlines. Using such an angledconfiguration makes the element relatively easy to draw and tends toreadily conform it with the airfoil shape. An angled outside corner suchas that shown in FIG. 7, however, promotes uneven heat distribution inoutside corners.

Referring to FIG. 8, a computer generated analysis of the current fluxpattern around corners 82, 84 is shown. As indicated by the graphicanalysis, in which current flux is represented by the stippled portion,current flux decreases dramatically toward outside corners 84. Hence,while cold spots cannot be eliminated in the outside corners 84 of tums54, the cold spots at least can be reduced by using sharp or squareoutside corners 84, as in FIG. 6. Employing perpendicular elementsegments with square outside corners 84, such as the ones in FIG. 6,allows for efficient arrangement of the elements 50 and results in aheat pattern with smaller cold spots in the outside corners 84. Incontrast to the rounded outside corners of elements 12 (FIG. 2), thecorners 84 define more conductive area to which heat can be transferred.In contrast to element 85 of FIG. 7, the corners 84 are moresymmetrical, and thus promote more even heat distribution therein.

Proper dimensioning of the elements 50 improves current flux density,and thereby improves performance of the de-icer 40. Computer-generatedanalysis has indicated that, in a preferred example, the width of thesegment 52, i.e. b (FIG. 6), and the inter-segmental gap, i.e. g,preferably are less than 0.055 inch and 0.045 inch, respectively.Moreover, for each conductive segment 52, current flux lines 83 are moreevenly distributed when the distance between the inside corner 82 of aturn 54 and the next adjacent inside corner 82 of the next adjacent turn54, i.e. w, is at least five times greater than b. Thus, heat isgenerated more uniformly in each conductive segment 52 having a w:b ofat least 5:1.

As can be recognized now, the dimensions of the elements 12 in FIG. 2are inadequate since the ratio between w and b is not maintained at avalue of at least five throughout. In contrast, all of the elements 50of FIG. 3 maintain at least a five to one ratio between w and b.

Referring to FIG. 9, the operation of de-icer 40 is explained in furtherdetail. In the example of FIG. 9, the de-icer 40 is disposed on asix-foot section of the structural member 11, and includes two spanwiseshedding areas 86, each of which includes one of the spanwise partingstrips 44 and a plurality of spanwise shedding zones 46. Six groups ofspanwise shedding zones 46 are shown in FIG. 2, and for convenience ofdiscussion, the grouped spanwise shedding zones 46 will be indicated bycapital letters, namely A-D. Additionally, in the preferred embodimenteach of the spanwise parting strips 44, as well as each of the spanwiseshedding zones 46, can be controlled individually.

In a preferred form of operation, current is transmitted continuously tothe spanwise parting strip 44, which typically is positioned near thestagnation line, and the chordwise parting strips 45. Accordingly, heatis generated continuously in the parting strips 44,45. Heat is generatedcontinuously in the spanwise parting strip 44 since it is difficult tode-ice areas adjacent to the stagnation line, where rime ice commonlyaccumulates. The temperature of the parting strips 44,45 preferably ismaintained above freezing, i.e. 32° F. To be conservative, thetemperature of the strips 44-45 preferably is maintained at about 50° F.or greater. As indicated in the timing chart of FIG. 10, zones A-D areheated sequentially to melt accumulated ice. Use of sequential heatingdecreases the amount of power consumed by the de-icer 40.

Typically, ice removal near the aft zones of the structural member 11 isless critical to lift than ice removal on forward zones of thestructural member 11. Consequently, as illustrated in FIG. 10, it may bedesirable to apply heat more frequently to spanwise groups A and B thanto spanwise groups C and D. For the example of FIGS. 9 and 10, currentpulses are applied to spanwise groups A and B during each power cycle.The power cycles are defined by eight increments, each of which may be4-12 seconds. The duration of each time interval is determined by, amongother features, ambient temperatures to which the spanwise groups A-Dare subjected, the efficiency of the de-icer 40 and the shape of thestructural member 42.

In the example of FIG. 9, since the contacts 51 and the end portions ofthe spanwise shedding zones 46 are overlapped by continuously heatedchordwise parting zones 45 (FIGS. 3 and 4), ice can accumulate only inspanwise groups A-D. Additionally, use of continuously heated chordwiseparting zones 45 insures that ice bridging cannot occur between thespanwise shedding areas 86. That is, even when spanwise group A beginsto re-ice during the time interval in which it receives no current, icebridging from spanwise group A to spanwise group B is avoided since icecannot transcend the heated chordwise parting zone 45 separating thegroups A and B. This bridging problem also could be solved byoverlapping the time intervals during which the spanwise groups A and Bare heated. This alternative solution might not always be desirable,however, since it inevitably increases power consumption. It can berecognized that such alternative solution would be particularlyapplicable in those arrangements in which the chordwise parting strips45 are not used.

In the example of FIG. 10, to fully melt the ice between the junctionsof spanwise groups A and C as well as B and D (FIGS. 9 and 10), heat wasapplied to spanwise groups C and D for two heating cycles. It isprobable that if elements 50 near the junctions of spanwise groups A andC (FIG. 3) as well as the junctions between spanwise groups B and D wereoverlapped only one heating cycle would have been required to affordeffective de-icing. Moreover, any ice bridging between groups A and C aswell as groups B and D could be minimized by overlapping the timeintervals during which groups A and C as well as groups B and D areheated.

As can be fully appreciated now, many of the interheater gaps 58 can beeliminated by overlapping the elements 50. Moreover the extent ofoverlap need not be significant to accomplish desirable heatdistribution patterns throughout each of the overlapped groups A-D. Thatis, simply by overlapping outer corners 84 of two elements 50, coldspots are substantially eliminated. When the outside corners 84 of twoelements 50 are superimposed (FIG. 11), the heat transfer to each ofelements 50 is improved considerably. For example, assuming that eachoutside corner 84 is heated at 2 BTU/ft² /hr, upon superimposing theoutside corners 84, the amount of heat flow in the vicinity of thesuperimposed corners 84 approximately is doubled to about 4 BTU/ft² /hr.

As long as the outside corners 84 are overlapped, as described above, animportant objective is achieved, i.e. to improve the heating capabilityof de-icer 10. It has been found, nonetheless, that when the elements 50are offset so that the outside corners 84 are overlapped, as in FIG. 11,an optimum heating pattern is achieved between the overlapped elements50. By offsetting the elements 50, the possibility of developing hotspots, particularly toward the middle sections of turns 54, isminimized.

As mentioned above, the spanwise parting strip 44 and each of thespanwise shedding zones 46 can be controlled individually. It has beenfound that this characteristic can be used to provide added flexibilityin de-icing. More particularly, the spanwise parting strip 44 usually ispositioned to coincide with the stagnation line that exists undertypical flight conditions; however, the stagnation line will shiftaccording to angle of attack of the aircraft. Any extensive change inthe angle of attack can shift the stagnation line into the vicinity ofthe spanwise shedding zones 46.

By controlling heating of the spanwise parting strips 44 and the variousspanwise shedding zones 46, the location of the spanwise parting strip44 can be shifted. That is, one of the spanwise shedding zones 46 can beheated continuously and designated as the spanwise parting strip 44.Similarly, the original spanwise parting strip 44 and the remainingspanwise shedding zones 46 can be heated intermittently and designatedas spanwise shedding zones 46. The shifted arrangement is maintaineduntil the stagnation line returns to its normal position. When thestagnation line returns to its normal position, continuous heating ofthe original spanwise parting strip 44 is resumed and intermittentheating of the temporary spanwise parting strip 44 is resumed.

In the preferred embodiment, the chordwise conductive segments 52 have alength of about one inch. Since the spanwise parting strip 44 and thespanwise shedding zones 46 can be controlled individually, the locationof the spanwise parting strip 44 (FIG. 9) can be shifted, in incrementsof one inch, above or below the commonly encountered stagnation line.

During normal flight conditions the angle of attack commonly varies. Theposition of the stagnation line shifts up or down as the angle of attackvaries. As mentioned above, the spanwise parting strip 44 desirably ispositioned about the stagnation line. It can therefore be appreciatedthat the ability to shift the spanwise parting strip 44 up or down inresponse to shifts in the stagnation line is an advantageous aspect ofthe de-icer 40.

In one example of operation, the spanwise shedding zones 48 are poweredintermittently at 20W/in² while the parting zones are poweredcontinuously at 8W/in² -30 W/in². These watt densities Will vary inresponse to a host of environmental and structural factors, such as windand temperature conditions to which the structural member 11 issubjected as well as the temperature at which the parting stripe 44,45and the spanwise shedding zones 46 are maintained.

In the foregoing description, it will be readily appreciated by thoseskilled in the art that modifications may be made to the inventionwithout departing from the concepts disclosed herein. Such modificationsare to be considered as included in the following claims unless thoseclaims, by their language, expressly state otherwise.

What is claimed is:
 1. A method for de-icing an aircraft structuralmember having a stagnation line developed during flight that moves froma first position to a second position resultant from a change in flightcondition, the method comprising the steps of:providing a firstelectrothermal heater coincident with the first position; providing asecond electrothermal heater coincident with the first position;providing a second electrothermal heater coincident with the secondposition; continuously heating the first electrothermal heater andintermittently heating the second electrothermal heater when thestagnation line is at the first position; and, intermittently heatingthe first electrothermal heater and continuously heating the secondelectrothermal heater when the stagnation line is at the secondposition.
 2. The method of claim 1, wherein the first electrothermalheater has a border including a first marginal portion, and the secondelectrothermal heater has a border including a second marginal portion,the first and second heaters being positioned relative to each othersuch that the marginal portions are overlapped.
 3. The method of claim2, wherein the heaters are attached to an outer surface of thestructural member and further comprising an insulating layer disposedintermediate the heaters and the outer surface of the structural member.4. The method of claim 1, wherein the first electrothermal heatercomprises a first electrically conductive strip having two ends, and thesecond electrothermal heater comprises a second electrically conductivestrip having two ends, each end including an enlarged area that definesan electrical contact, and each of the electrical contacts is covered bythe first heater or the second heater.
 5. The method of claim 2, whereinthe first heater and the second heater each include:a plurality ofelongate, electrically conductive members disposed generallyperpendicular to a common axis, each of the conductive members having afirst end and a second end, and each of the conductive members beingspaced from the adjacent conductive members by an inter-segmental gap;and a plurality of electrically conductive turn portions, the turnportions interconnecting the first ends and the second ends of theconductive members to form a ribbon.
 6. The method of claim 5, whereinthe conductive members are about 0.06 inch wide.
 7. The method of claim5, wherein the inter-segmental gaps are less than about 0.05 inch wide.8. The method of claim 5, wherein a portion of each conductive memberbounded by two of the turn portions has a length that is at least fivetimes greater than a width of the conductive member.
 9. A method forde-icing an aircraft structural member having a stagnation line thatchanges position resultant from change in flight condition, the methodcomprising the steps of:providing a first electrothermal heater disposedat a first position on the structural member; providing a secondelectrothermal heater disposed at a second position on the structuralmember; continuously heating the first electrothermal heater andintermittently heating the second electrothermal heater when thestagnation line is located in the vicinity of the first position; and,intermittently heating the first electrothermal heater and continuouslyheating the second electrothermal heater when the stagnation line is inthe vicinity of the second position.
 10. The method of claim 9, whereinthe first electrothermal heater has a border including a first marginalportion, and the second electrothermal heater has a border including asecond marginal portion, the first and second heaters being positionedrelative to each other such that the marginal portions are overlapped.11. The method of claim 10, wherein the heaters are attached to an outersurface of the structural member and further comprising an insulatinglayer disposed intermediate the heaters and the outer surface of thestructural member.
 12. The method of claim 9, wherein the firstelectrothermal heater comprises a first electrically conductive striphaving two ends, and the second electrothermal heater comprises a secondelectrically conductive strip having two ends, each end including anenlarged area that defines an electrical contact, and each of theelectrical contacts is covered by the first heater or the second heater.13. The method of claim 10, wherein the first heater and the secondheater each include:a plurality of elongate, electrically conductivemembers disposed generally perpendicular to a common axis, each of theconductive members having a first end and a second end, and each of theconductive members being spaced from the adjacent conductive members byan inter-segmental gap; and a plurality of electrically conductive turnportions, the turn portions interconnecting the first ends and thesecond ends of the conductive members to form a ribbon.
 14. The methodof claim 13, wherein the conductive members are about 0.06 inch wide.15. The method of claim 13, wherein the inter-segmental gaps are lessthan about 0.05 inch wide.
 16. The method of claim 13, wherein a portionof each conductive member bounded by two of the turn portions has alength that is at least five times greater than a width of theconductive member.
 17. A de-icer adapted for attachment to a structuralmember having a stagnation line that changes position resultant fromchange in flight condition, comprising:a first electrothermal heaterdisposed at a first position on the structural member; a secondelectrothermal heater disposed at a second position on the structuralmember; wherein said first electrothermal heater is continuously heatedand the second electrothermal heater is intermittently heated when thestagnation line is located in the vicinity of the first position, andthe first electrothermal heater is intermittently heated and the secondelectrothermal heater is continuously heated when the stagnation line isin the vicinity of the second position.
 18. The de-icer of claim 13,wherein the first electrothermal heater has a border including a firstmarginal portion, and the second electrothermal heater has a borderincluding a second marginal portion, the first and second heaters beingpositioned relative to each other such that the marginal portions areoverlapped.
 19. The de-icer of claim 18, wherein the heaters areattached to an outer surface of the structural member and furthercomprising an insulating layer disposed intermediate the heaters and theouter surface of the structural member.
 20. The de-icer of claim 17,wherein the first electrothermal heater comprises a first electricallyconductive strip having two ends, and the second electrothermal heatercomprises a second electrically conductive strip having two ends, eachend including an enlarged area that defines an electrical contact, andeach of the electrical contacts is covered by the first heater or thesecond heater.
 21. The de-icer of claim 18, wherein the first heater andthe second heater each include:a plurality of elongate, electricallyconductive members disposed generally perpendicular to a common axis,each of the conductive members having a first end and a second end, andeach of the conductive members being spaced from the adjacent conductivemembers by an inter-segmental gap; and a plurality of electricallyconductive turn portions, the turn portions interconnecting the firstends and the second ends of the conductive members to form a ribbon. 22.The de-icer of claim 21, wherein the conductive members are about 0.06inch wide.
 23. The de-icer of claim 21, wherein the inter-segmental gapsare less than about 0.05 inch wide.
 24. The de-icer of claim 21, whereina portion of each conducive member bounded by two of the turn portionshas a length that is at least five times greater than a width of theconductive member.